1. Field of the Invention.
The present invention pertains to apparatus and methods for cementing liners in well bores. More specifically, it pertains to apparatus which permits cementing of liners in well bores without some of the attendant pressure problems of the prior art.
2. Brief Description of the Prior Art.
A liner is a section of casing or tubing which is usually suspended in a well casing without normally extending to the surface. Cemented liners are utilized for a number of reasons: providing well control, reducing initial cost of casing, more rapid installation than full casing strings, etc. Liners may be installed entirely within outer casing strings or partially within an open hole.
Conventionally, a liner is set and cemented by first lowering the liner and a setting tool, connected to an operating string, into the well bore. The liner is hung, usually on slips, and the setting tool is released from the liner. Then cement is pumped through the operating string, into the liner, and displaced from the liner, usually through a foot valve, into the annular space between the liner and surrounding casing or well bore.
Usually, at least one wiper plug is introduced into the liner string immediately behind the cement in order to separate the cement from the displacing fluid and to wipe cement from the liner surface as the cement is pushed out of the liner into the surrounding annular space. It is also conventional to utilize at least one additional plug for introduction into the operating string to separate the cement from the displacing fluid and to wipe the cement from the operating string as cement is pumped into the liner. In more recent cementing operations, the plug which is to wipe the operating string is pumped behind the cement until it engages the liner wiper plug and then both plugs are forced downwardly together in the liner string so as to displace cement therefrom and to wipe the liner walls. Such apparatus may be seen in U.S. Pat. No. 3,364,996 - C. C. Brown and U.S. Pat. No. 3,635,288 - Lebourg.
In such cementing operations, it is also necessary to provide some sort of seal in the annular space between the setting tool and the surrounding liner since fluid pressure must be maintained in the operating string during the cementing operation. It has been common to provide this seal by attaching chevron or inverted swab cup type seal assemblies to the setting tool mandrel for sealing engagement with the surrounding liner at some point above the liner wiper plug. However, once the pumpdown plug has engaged the wiper plug and both plugs are forced downwardly through the liner to displace the cement therein, the pressure applied thereto acts against the lower end of the mandrel and the seal assembly applying an upwardly directed force tending to force the operating string out of the well. Therefore, a substantial amount of weight must be maintained on the operating string to prevent this from occurring.
To reduce the forces which tend to force the operating string out of the well, the seal assembly has been directly connected in the liner string. In such cases, the setting tool is provided with a "slick" stinger or mandrel for sealing engagement with an annular seal provided in a central opening of the seal assembly. Thus, the forces acting upwardly on the seal assembly during cementing are transferred to the liner rather than the operating string. However, once cementing has been completed, such a seal assembly must be drilled out. This requires additional time and money and may result in problems.